Enhanced E-layer ionization in the auroral zones observed by radio occultation measurements

Kurzfassung

Electron precipitation of magnetospheric origin causes additional ionization in the auroral zone at E-layer heights. In the sun-lighted ionosphere this type of ionization contributes to the total ionization to a certain degree. During night-time, in particular at winter-night, the E-layer ionization may dominate over the F2 layer ionization level. To study the geophysical conditions and characteristics of the related ionospheric processes in more detail, we use GPS radio occultation electron density profile retrievals from CHAMP and COSMIC to extract those vertical profiles which show the absolute maximum of ionization in the E-layer height range of 90 – 150 km. In order to select these profiles, we have developed an algorithm which can recognize the shape of a given profile by fitting an empirical Ansatz to it. The upper part of the profile, from hmF2 - 40km to maximally 800 km, is fitted to the weighted sum of a Chapman layer function and an exponential function for the topside. The E-Layer, if existing, is similarly fitted to another Chapman layer function. Our data basis consists of COSMIC profiles from 2006.330 – 2007.060 and of CHAMP profiles from 2001.300 to 2007.060, allowing us to study both, the local-time dependence and the solar-cycle variability of the observed processes. It has to be mentioned that the retrievals from CHAMP and COSMIC provide consistent results. The observations from both data sets clearly show the location of these events being closely related to the auroral oval around the geomagnetic pole. As expected, the number of such events increases with enhanced geomagnetic activity